Review on the Nitration of [60]Fullerene, CHEMIA I PIROTECHNIKA, Chemia i Pirotechnika

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Propellants, Explosives, Pyrotechnics 26, 109 – 111 (2001)
109
Review on the Nitration of [60]Fullerene
Nai-Xing Wang*
Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100080, (PR China)
Summary
methods, including the thermal decomposition of heavy
metal nitrate, the oxidation of nitric acid, the reaction of
dinitrogen trioxide with dinitrogen pentoxide, the reaction of
nitric acid with sulfur dioxide, phosphorus pentoxide or
copper, the reaction of nitrosyl chloride with silver nitrate,
and the reaction of nitrosyl hydrogen sulfate with potassium
nitrate
(12)
. The brown colored nitrogen dioxide exists in
equilibrium with its dimer dinitrogen tetroxide N
2
O
4
,
which is colorless. At ambient temperature the ratio of
N
2
O
4
=NO
2
is about 86
:
16
(13)
. The equilibrium content of
NO
2
increases as the temperature rises and reaches 99% of
an NO
x
mixture at 135
C
(12)
. Nitrogen dioxide is a reactive
intermediate for nitration reactions of many organic olefins.
The reaction is normally initiated by the attack of a nitronium
ion, which is generated by the release of the unpaired electron
from
NO
2
, to an olefinic double band.
It has been suggested that C
60
acts like a closed-cage
alkene rather than an aromatic molecule due to its poor
electron delocalization
(14)
. For example, C
60
undergoes
many organic reactions, such as reactions including various
addition reactions. The additions can be categorized into
additions of halogens
(15 – 17)
, Diels-Alder cycloaddition
(18)
and radical addition
(19)
. The radical addition is the main
chemical reaction of C
60
due to its high electron affinity
allowing a direct chemical attack by the radicals. It has
been proved that the C
60
molecule is high reactive towards
radical species
(20,21)
and its reactivity can be much greater
than that of a typical olefin towards the same radical.
Therefore, radical
NO
2
is an excellent reactivity group to
make direct multiplet additions with [60]fullerenes to get
polynitrofullerenes.
How to get as much as possible of
NO
2
radical? Recently,
L. Y. Chiang et al.
(22)
reported nitration experiments and
some methods for the generation of nitrogen dioxide which
was obtained by the reaction of sodium nitrite with conc.
HNO
3
or from the reduction of conc. HNO
3
by copper
powder. Under their conditions, an instantaneous production
of red-brown
NO
2
gas was observed due to a fast reaction of
Cu with N
2
O
4
.
[60]Fullerenes continued to attract the interest of a broad scientific
community since the method of macroscopic synthesis of fullerenes
was discovered in 1990. Studies on the nitration of fullerenes have
been carried out under different conditions and nitration reagents. The
radical (
NO
2
) is an excellent reactivity group to make direct multiplet
additions with [60]fullerene to get polynitrofullerenes. Radical (
NO
2
)
can be obtained by many different methods.
1. Introduction
Since the discovery of C
60
and its large scale prepara-
tion
(1,2)
a lot of work has been devoted to this research
field
(3–5)
.
C
60
has a good anticompression performance, therefore,
C
60
can become an excellent solid rocket fuel additive to bear
a huge pressure in the propellants. Some energetic groups
such as trinitrophenyl were lead into C
60
to get a better new
fuel additive
(6)
.
The nitration of fullerene molecules has been carried out
under different conditions and nitration reagents, including a
mixture of conc. HNO
3
and sodium nitrite
(7)
, dinitrogen
tetroxide
(8)
, fuming nitric acid
(9)
, and a mixture of aqueous
sodium nitrite, FeSO
4
, and H
2
SO
4
in the presence of air
(10)
.
However, polynitrofullerenes react slowly with H
2
O to yield
partially hydroxylated products of poly(hydroxynitro)-
fullerenes. In the presence of an aqueous alkaline solution a
rapid and complete hydrolysis of polynitrofullerenes was
observed to produce fullerenol molecules containing at least
16 hydroxy groups per C
60
cage
(7)
.
2. Nitration Agent
The nitronium ion (NO
2
) as a nitration agent was first
reported in the reaction with fullerene molecules forming
reactive polynitrofullerenic carbocation intermediates
(11)
.
For the synthesis of separable nitrofullerene derivatives,
nitrogen dioxide radical (
NO
2
) was the commonly used
nitration reagent. It was generated by a number of preparative
3. Development
* e-mail: naixingwang@hotmail.com
During the NO
2
=N
2
O
4
generation
NO
2
is the only gaseous
product available among several other possible nitrogen
oxide species, such as NO
þ
,NO
2
,NO
ð23Þ
2
. The progress of
#
WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001
0721-3115/01/0306 – 0109 $17.50
þ:
50
=
0
110 Nai-Xing Wang
Propellants, Explosives, Pyrotechnics 26, 109 – 111 (2001)
[NO ]
2
4
[NO ]
2
6
Figure 1. Nitration of fullerene.
C
60
nitration was monitored by the disappearance of the
purple color of solution of C
60
and the formation of the red
polynitrated fullerene product.
The nitration rate depends on both the temperature and the
concentration of [
NO
2
]. The completion of the reaction was
achieved within 1.0 h at 160
C or 8 days at 15 – 20
C.
It is known that a chemical equilibrium is built between
NO
2
and N
2
O
4
.
2
NO
2
,
N
2
O
4
In 1992, Chiang et al. reported the versatile nitronium
chemistry for C
60
fullerene functionalization
(10)
and then in
1996 they reported that the C
60
molecule exhibited high
reactivity towards the addition reaction of nitrogen dioxide
radical (
NO
2
). NO
2
was generated by the reaction of sodium
nitrite with conc. HNO
3
and this chemical functionalization
of C
60
resulted in polynitro fullerene C
60
ðNO
2
Þ
ð7
x
. Sakar
et al.
(9)
reported that C
60
was nitrated by the multiple
addition of
NO
2
and the product isomerized partly to the
nitrito form with subsequent hydrolysis by atmospheric
moisture to yield nitrofullerols consisting of 6–8 nitro and
7–12 hydroxy groups per C
60
. In their method
(9)
NO
2
was
passed through a toluene solution of C
60
, resulting in a color
change to deep orange within 5min at ambient conditions.
Addition of an excess of light petroleum (40 – 60
C) or
n-hexane precipitated a yellow-brown solid in high yield
with an average of 6–8 nitro and 7–12 hydroxy groups per
C
60
molecule. The IR spectra of the solid showed typical NO
2
vibrations at 1560, 1336 and 809 cm
7
1
. In 1993 the author
used a lot of time for the research on [60]fullerene and some
of its derivatives
(29)
. He thinks that C
60
actually is an active
carbon cage and it reacts very easily with radical ions. It is not
difficult for energetic materials chemists to get
NO
2
radicals.
Experimental conditions were set up under inert atmosphere
to obtain a higher yield of polynitrofullerene which is an
excellent research field.
A pressure increase is not advantageous for the generation
of radical [
NO
2
]. When
NO
2
was injected into the reaction
medium containing C
60
molecules, its self-ionization into
NO
2
and NO
2
is known to be a slow process
(24)
. This allows
to count on the nitrogen dioxide radicals as the main species
in the reaction with C
60
. An odd-numbered addition of
NO
2
radicals to a C
60
molecule may get a net residual radical on
the C
60
, polynitrated fullerene products that allow to char-
acterize these products unlike C
60
containing an even number
of nitro groups
(22)
.
Fullerene nitro functional groups were found to be some-
what thermally unstable in solution or on SiO
2
which
prohibits the direct purification and separation of polyni-
trated isomers under prolonged chromatographic conditions.
Each tetranitro[60]fullerene isomer comprising 56 conju-
gated olefinic carbons should display at least 28 or 56
aromatic carbon peaks if the compound exhibits either a
twofold or no structural symmetry. However, as the number
of regioisomers increases the total number of aromatic
carbon peaks increases rapidly, which is an isomers
mixture and often results in a broad band of carbons centered
roughly at
d
145. The first fullerene derivative to be made was
C
60
H
36
ð25Þ
but paradoxically its structure has remained
unresolved. Just in theoretical works the structures of four
C
60
H
36
isomers with the symmetry T, T
h
,D
3d
and S
6
were
considered. The structure with T symmetry contains four
isolated benzenoid rings located in the tetrahedral positions
on the surface of the closed skeleton of the molecule. The
structure of the T
h
symmetry contains 12 isolated double
bonds in five-membered rings. Fullerene hydrides with the
structures of the D
3d
and S
6
symmetry have one benzenoid
ring at each pole of the molecule and isolated double bonds
along and parallel (D
3d
) as well as perpendicular (S
6
) to the
equator of the molecule. According to the calculations, the
C
60
H
36
structure with T symmetry is the most stable one (see
Refs. 26 – 28). If we obtain C
60
(NO
2
)
36
, the same problem
will be met as C
60
H
36
. The number of regioisomers probably
increases more than C
60
H
36
. But C
60
(NO
2
)
x
as a new
energetic material does not matter about those regioisomers.
4. References
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Propellants, Explosives, Pyrotechnics 26, 109–111 (2001)
Review on the Nitration of [60]Fullerene 111
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(Received October 11, 1999; revised January 11, 2001;
Ms 1999
=
64)
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